Conventional lift cranes include a upper body or upper works which is typically rotatable about a vertical axis with respect to a lower works or carbody. The tipper works typically supports a boom or mast and carries lift machinery for raising and lowering the boom as well as machinery for operating the lifting hoist. The carbody in mobile cranes, typically includes a crawler-type drive system including a pair of track assemblies driven by a suitable drive motor or motors.
The lifting capacity of a crane is determined at least in part by the geometry of the base, since all of the compression and tilting loads must act through the base which is in contact with the ground. The need for increased crane capacity has been partly met by larger sized cranes having bigger carbodies, both for more strength and to further space the fulcrum or tipping point of the crane from the counterweight effective line of action. Larger cranes have also been provided with increasing amounts of counterweight carried on the rotatable upper works which resist the overturning moment of larger loads.
Cranes having auxiliary counterweights and/or movable counterweight mechanisms have been suggested in the past. U.S. Pat. No. 5,035,337 illustrates an example of a crane having auxiliary counterweights which are used in order to lift heavy loads. U.S. Pat. No. 4,557,390 illustrates a suspended counterweight system in which a separate counterweight is supported by a separate counterweight boom..
U.S. Pat. Nos. 4,729,486 and 4,953,722 illustrate mechanisms for providing a counterweight that is movable along a beam extending from the rear of the upper works. In the '486 patent, an actuator operating a chain drive is provided for selectively moving the counterweight along the beam is disclosed.
Large capacity mobile cranes often have to be disassembled in order to transport the crane to a job site. In some instances, the upper works is separated from the carbody and the individual units are transported to the job cite and reassembled. In at least some large capacity cranes, the counterweight is also separately transported and is installed onto the upper works at the job site. Attempts have been made to simplify the installation of the counterweight. U.S. Pat. No. 4,081,081 illustrates an example of a crane in which provision has been made for mounting and demounting a counterweight.